I would put the item to one side for now and wait and see if the seller realises the mistake they have made.
If they do ask for it to be returned, they will need to provide a label.
Was it addressed to you by name? The seller could (as suggested) have two accounts and may have just got a bit muddled when printing labels.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)